batman-adv: Avoid WARN on net_device without parent in netns

BugLink: https://bugs.launchpad.net/bugs/1818813

commit 955d3411 upstream.

It is not allowed to use WARN* helpers on potential incorrect input from
the user or transient problems because systems configured as panic_on_warn
will reboot due to such a problem.

A NULL return value of __dev_get_by_index can be caused by various problems
which can either be related to the system configuration or problems
(incorrectly returned network namespaces) in other (virtual) net_device
drivers. batman-adv should not cause a (harmful) WARN in this situation and
instead only report it via a simple message.

Fixes: b7eddd0b ("batman-adv: prevent using any virtual device created on batman-adv as hard-interface")
Reported-by: syzbot+c764de0fcfadca9a8595@syzkaller.appspotmail.com
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

net/batman-adv/hard-interface.c

@@ -18,7 +18,6 @@
 #include "hard-interface.h"
 #include "main.h"
 
-#include <linux/bug.h>
 #include <linux/byteorder/generic.h>
 #include <linux/errno.h>
 #include <linux/fs.h>
@@ -104,8 +103,10 @@ static bool batadv_is_on_batman_iface(const struct net_device *net_dev)
 	/* recurse over the parent device */
 	parent_dev = __dev_get_by_index(&init_net, dev_get_iflink(net_dev));
 	/* if we got a NULL parent_dev there is something broken.. */
-	if (WARN(!parent_dev, "Cannot find parent device"))
+	if (!parent_dev) {
+		pr_err("Cannot find parent device\n");
 		return false;
+	}
 
 	ret = batadv_is_on_batman_iface(parent_dev);